On Mon, Jul 03, 2017 at 06:13:38PM +0100, Will Deacon wrote:

On Mon, Jul 03, 2017 at 09:40:22AM -0700, Linus Torvalds wrote:

quoted

On Mon, Jul 3, 2017 at 9:18 AM, Paul E. McKenney
[off-list ref] wrote:

quoted

Agreed, and my next step is to look at spin_lock() followed by
spin_is_locked(), not necessarily the same lock.

Hmm. Most (all?) "spin_is_locked()" really should be about the same
thread that took the lock (ie it's about asserts and lock debugging).

The optimistic ABBA avoidance pattern for spinlocks *should* be

    spin_lock(inner)
    ...
    if (!try_lock(outer)) {
           spin_unlock(inner);
           .. do them in the right order ..

so I don't think spin_is_locked() should have any memory barriers.

In fact, the core function for spin_is_locked() is arguably
arch_spin_value_unlocked() which doesn't even do the access itself.

Yeah, but there's some spaced-out stuff going on in kgdb_cpu_enter where
it looks to me like raw_spin_is_locked is used for synchronization. My
eyes are hurting looking at it, though.

That certainly is one interesting function, isn't it?  I wonder what
happens if you replace the raw_spin_is_locked() calls with an
unlock under a trylock check?  ;-)

							Thanx, Paul